TDOA/FDOA techniques have been employed in the past to determine the geolocation of emitters of electromagnetic radiation, such as radio frequency (“RF”) emissions. Using such techniques, the time difference (TDOA) in reception of a radio signal received at the sensing platforms of two spaced apart aircraft may be used to define a hyperboloid upon which the RF emitter of interest is located. Frequency difference (FDOA) in reception of the same radio signal at the sensing platforms of the two spaced apart aircraft may be generated by flying the spaced apart aircraft at different velocities and headings relative to the RF emitter, and may be used to further resolve the geo-location of the radio emitter. In this regard, assuming that the RF emitter of interest is located on the surface of the earth, the intersection of the TDOA hyperboloid, the FDOA surface, and the surface of the earth may be employed to determine a set of possible geo-locations for the RF emitter. However, only one of these possible geo-locations is real, the other possible geo-locations in the set are purely mathematical solutions that are not the real location of the radio emitter. To determine the correct geo-location requires additional information. This additional information is typically obtained from a system that can produce a line of bearing, from a third aircraft that can measure an independent TDOA/FDOA set of data, or by repositioning the two aircraft and then measuring a second TDOA/FDOA set of data.
Two aircraft have been employed as moving sensing platforms where sufficient time exists for measuring more than one set of TDOA/FDOA data from the RF emitter. This has been done by measuring a first set of TDOA/FDOA data at a first set of locations of the two aircraft followed by flying the two aircraft to a second set of locations and measuring a second set of TDOA/FDOA data at the second and new aircraft locations. The recomputed set of TDOA/FDOA data from the second set of aircraft locations may then be compared to the set of TDOA/FDOA data computed at the first set of aircraft locations to obtain a single overlapping answer for the geo-location of the emitter. Two aircraft may also be employed as moving sensing platforms in combination with additional equipment that is capable of generating a line of bearing, i.e., Inertial Navigation System (INS)/Global Positioning System (GPS) equipment, precision time reference equipment, and data link equipment.
TDOA/TDOA and FDOA/FDOA techniques have also been employed in the past to determine the geolocation of emitters of electromagnetic radiation, such as radio frequency (“RF”) emissions. For example, using a TDOA/TDOA technique, the time difference (TDOA) in reception of a radio signal received at the sensing platforms of three spaced apart aircraft may be used to define two hyperboloids upon which the RF emitter of interest is located. The intersection of the two TDOA hyperboloids and the surface of the earth may be employed to determine a set of possible geo-locations for the RF emitter, which may be further resolved as previously described above. In another example, using a FDOA/FDOA technique, frequency difference (FDOA) in reception of the same radio signal at the sensing platforms of three spaced apart aircraft may be employed to generate two FDOA curves, and the intersection of the two TDOA hyperboloids and the surface of the earth may be employed to determine a set of possible geo-locations for the RF emitter, which also may be further resolved as previously described above.
In a radio communication environment, multiple RF signal emitters may transmit over the same frequency and at the same time. However, typical conventional geolocation systems are configured to operate with the assumption that there is only one emitter on the selected frequency at any given time. When operating under this assumption, a network of multiple emitters transmitting on the same frequency actually act to interfere with each other and the conventional geolocation system will yield no solution or only an invalid solution. One example of such a network of multiple emitters is a group of push-to-talk (PTT) radio users. To enhance the situational awareness, it is highly desirable to detect and geolocate all radio users sharing a common frequency channel setting. One major obstacle to accomplishing this goal is the short up-time characteristics of these PTT signals. Most conventional single collection systems will not have sufficient time to detect the signal activity, queue, collect, and process for emitter geolocation. Even if the signal can be detected, there isn't enough good signal captured to allow computation of an accurate geolocation or the determination of the number of users on the selected frequency channel setting. Another obstacle to accurate geolocation and number determination is the close proximity of the users in a PTT network given the line-of-sight limitations of these types of radios.